1. Field of the Invention
The present invention relates to the pixel circuits and drive method of an active matrix display comprising light-emitting devices that emits light by conducting a driving current through a light emitting material such as an organic semiconductor thin film. Such pixel circuits comprise active elements, such as thin film transistors, for controlling the light emitting operation of the respective light emitting devices. More specifically, the present invention provides pixel circuits comprising an active conducting channel between a data input electrode and a scan electrode, and a method to operate such pixel circuits. Furthermore, pixel circuits in the present invention are structured with alternating conducting channels, controlled by a multi-functional control electrode. Pixel circuits capable of performing current-controlled drive scheme, with reduced complexity than existing solutions, are provided as preferred application of the present invention.
2. Description of the Prior Art
Organic light emitting diode displays (OLED) have attracted significant interests in commercial application in recent years. Its excellent form factor, fast response time, lighter weight, low operating voltage, and prints-like image quality make it the ideal display devices for a wide range of application from cell phone screen to large screen TV. Passive OLED displays, with relatively low resolution, have already been integrated into commercial cell phone products. Next generation devices with higher resolution and higher performance using active matrix OLEDs are being developed. Initial introduction of active matrix OLED displays have been seen in such products as digital camera and small portable video devices. Demonstration of OLED displays in large size screens further propels the development of a commercially viable active matrix OLED technology. The major challenges in achieving such a commercialization include (1) improving the material and device operating life, and (2) reducing device variation across the display area. Several methods have been suggested to address the second issue by including more active switching devices in individual pixels, by switching of power supply lines externally, or by reading back the pixel parameters combined with an external memory and tuning circuit. As more elaborated control circuits being incorporated into individual pixels as proposed in these solutions, concerns over complexity and practical manufacturing issues arise.
The operation of an OLED display differs from a liquid crystal display (LCD) in that each and every pixel in an OLED display comprises a light emitting element. The light output of such light emitting elements is more conveniently controlled by the current directed to the pixel. In contrast, an LCD is readily operable by voltage signals as its optical response being more favorably expressed in a simple form of applied voltage. While typical storage devices hold information in the form of voltage, operating an active matrix OLED display via a typical storage element requires a conversion mechanism within a pixel to convert a stored voltage data into specific current output. In practice, a conversion method needs to be reliable and fairly independent of such factors as pixel-to-pixel variation in the characteristics that affect said conversion, to make an OLED display operable with fair uniformity.
Basic examples of using organic material to form an LED are found in U.S. Pat. No. 5,482,896, U.S. Pat. No. 5,408,109 and U.S. Pat. No. 5,663,573, and examples of using organic light emitting diode to form active matrix display devices are found in U.S. Pat. No. 5,684,365 and U.S. Pat. No. 6,157,356, all of which are hereby incorporated by reference.
An active matrix OLED display (FIG. 1) is typically structured with “SELECT” electrodes for row select, “DATA” electrodes for setting the pixel state, power electrodes VDD to drive the pixels, and a reference voltage VREF to provide a common voltage level. A basic pixel in an active matrix display also comprises at least one transistor for data control, and at least a storage element to hold the data information sufficiently long so a pixel remains stable in a data state in an image frame. A circuit diagram for a basic pixel 100 in an active matrix OLED display is depicted in FIG. 2 in further detail. An active matrix display with pixel circuit structured as in FIG. 2 allows data to be written and retained in a storage capacitor 204 according to the data signal delivered from a data electrode in an address cycle, while the power supply VDD continuously drives OLED 205 through an n-channel transistor 201, according to the data setting in capacitor 204. The selection of pixels to receive data information is controlled by an n-channel transistor 203 that is controlled by the voltage on a select electrode connected to the gate of transistor 203. An active matrix driving scheme allows the drive transistor 201 remain in a data state, and continue to deliver the required drive current, for an extended period of time after the input data on the data electrode is disconnected from the pixel. The peak current required for achieving a certain brightness level is thus reduced accordingly compared to a passive matrix. The peak driving current in an active matrix display does not scale with the resolution as in a passive matrix, making it suitable for high resolution applications. Stability of the active matrix display is also improved appreciably.
As illustrated in the above example, the electrical current for producing light output is directed to the light emitting element via a current path that comprises at least a control element that regulates the current. In a conventional light emitting device display, these control elements are fabricated on a thin film of amorphous silicon on glass. Power consumed in such control elements are converted to heat rather than yielding any light. To reduce such power consumption, polycrystalline silicon is preferred over amorphous silicon for its better mobility. More elaborated methods employing self-regulated multiple-stage conversions suitable for pixel circuit using polysilicon base material may be found in U.S. Pat. No. 6,501,466 and U.S. Pat. No. 6,580,408. These methods provide a current drive scheme while largely eliminated the impact from material and transistor non-uniformity typically associated with thin film polysilicon on glass base plate. In these methods, typically a minimum of four transistors are required to achieve such self-regulated, multi-stage conversion to achieve a pixel-independent current drive for the light emitting device display. An example of such methods is illustrated in FIG. 3. where four transistors 301, 302, 303, and 307, and 3 access electrodes, DATA, SELECT, and VDD, are used for each pixel with a storage capacitor 304 and an OLED 305.
The circuit in FIG. 4 illustrates another method for a self-regulating current drive scheme. The display circuit includes a switch on a power supply electrode, switching the source voltage between two voltage levels VDD1 and VDD2. Comparing to the example of FIG. 3, the transistor count of FIG. 4 is less than that of FIG. 3, but an additional access electrode with switching capability is required to operate the pixel and to deliver drive current to the light emitting diode in a current drive scheme.
FIG. 5 illustrates another method that reads the pixel parameters into an external processing circuit that comprises memory and adjustment circuitry. The variations of pixel parameters, such as the threshold voltage variation, may be eliminated by such external adjustment. The pixel circuit comprises five transistors and five access electrodes.
These examples of prior art provide a brief overview of the existing solutions considered in the art to resolve the uniformity issue. Comparing to the basic pixel circuit in FIG. 2, it is evident that any current solution to the uniformity issue involves a substantial increase in the complexity of pixel circuit, and thus likelihood of reduction of available light emitting area, efficiency, and product yield.
The present invention provides a multi-functional scan electrode for pixel access that carries the conventional pixel select function and providing a conversion function for converting a data current to a data voltage. The present invention further provides multiple conducting channels in a pixel, for setting the data voltage and delivering drive current. The pixel structure so constructed comprises a direct current path from a data electrode to a scan electrode, and may further comprise a direct current path from a scan-power electrode to the light emitting element. The turning-on and off of such channels are fully controlled by the voltage applied on a scan-power electrode.